The present invention relates to disposable heating units for use for heating the contents of a container.
It is known to provide self-contained, self-heating devices for quickly heating liquid foods such as beverages, soups and the like, in circumstances in which conventional heat sources are either unavailable or inconvenient.
For ease of presentation, the devices may be classified according to the mechanism used for heat generation into four groups. A first group employs hydration processes, a second employs acid-base reactions. The third employs spontaneous oxidation-reduction reactions in aqueous solutions, and a the fourth employs kinetically non-spontaneous oxidation-reduction reactions between solid oxidant and a solid reductant. Each group will now be addressed in turn.
Turning firstly to devices employing hydration processes, these devices generate heat by adding water onto an ionic solid, such as calcium oxide or calcium chloride. Examples of this type may be found in U.S. Pat. No. 5,626,022 to Scudder et al., U.S. Pat. No. 5,388,565 to Ou, and U.S. Pat. No. 4,773,389 to Hamasaki.
These devices have been found relatively simple to implement and use. However, the hydration processes used suffer from a number of inherent limitations. Firstly, the specific heat of the reactions employed is relatively low (roughly 100 Cal/ml), requiring the use of a large storage volume to provide a given heating effect. This problem is exacerbated by the significant heat energy which is absorbed by heating up the aqueous phase. Furthermore, the presence of significant quantities of water inherently limits the temperature of the heating unit to 100.degree. C. such that the liquid food rarely reaches in excess of about 80.degree. C.
A second approach employs mixing of acids and bases. A recent patent that takes advantage of this approach is U.S. Pat. No. 5,935,486 to Bell et al. that involves mixing of various organic and inorganic acids and bases.
The third approach uses oxidation-reduction reactions occurring in the aqueous phase. Examples of this type include U.S. Pat. No. 5,517,981 to Taub et al. in which magnesium is mixed with cupric chloride in the presence of water and the U.S. Pat. No. 3,998,749 to Hydro et al. where aluminum and cupric chloride are mixed in a mixture of aqueous and organic solvents.
Oxidation reactions of this type are highly exothermic, providing greater heat per unit storage volume than hydration reactions. However, the use of such reactions also presents certain problems. Firstly, the reactions tend to progress very rapidly, making it difficult to ensure efficient heat transfer to the liquid food. Furthermore, because of the need for the presence of some water, substantial energy is wasted in heating the water or boiling part of it. Finally, most reactions in this group produce significant quantities of dangerous gases such as hydrogen, and the waste solution may include hazardous substances, leading to numerous safety and environmental problems.
A fourth group of devices achieve significant advantages of efficiency, simplicity of structure and controllability by using solid phase self-propagating high-temperature synthesis (SHS) reactions, which include oxidation-reduction processes in the solid-state (such as thermite reactions). These reactions are basically redox reactions between metals or semimetals and metal oxide, such as aluminum, silicon and ferric oxide. In addition, these reactions are gas-less processes that involve harmless materials, and generate large amounts of heat per unit volume (or weight) of the reagents. The temperature of SHS reactions is above 1000.degree. C., which requires good heat transfer and a safe metallic inner container. The rate of reaction may be controlled by appropriate choice of metals and metal oxides, grain size of the solids and path of reaction. Since the reactions are not kinetically spontaneous at room temperature, the components may be safely and conveniently mixed within a single chamber until activated by the user.
Self-heating devices based on various thermite reactions have been proposed. Examples include U.S. Pat. Nos. 4,506,654 to Zellweger et al., U.S. Pat. No. 4,819,612 to Okamoto et al., U.S. Pat. No. 4,949,702 to Suzuki et al., U.S. Pat. No. 5,020,509 to Suzuki et al., and U.S. Pat. No. 5,220,908 Iizuna et al. In all of these examples, the fuel is a mixture of a metal or alloys, such as silicon or ferrosilicon and a metal oxide, such as ferric oxide or cupric oxide.
Despite all of the above-mentioned advantages of solid-phase oxidation reactions, implementation of self-heating devices using these reactions is complicated by the need for an ignition system. Actuation of the chemical reaction is typically achieved by means of friction (similar to a match), by an electric ignition, or by manual ignition of an external fuse.
There is therefore a need for a self-heating-container assembly and heating unit for use therewith which would employ a solid-phase oxidation reaction without requiring the use of a complicated and expensive ignition system. It would also be highly advantageous to provide a method for heating the contents of a container according to which a primary solid-phase oxidation reaction would be initiated by an exothermic initiation reaction which provides the initiation energy by mixing small amounts of reagents which undergo a spontaneous exothermic chemical reaction.